Thirty years ago, the pineal was known only as a tissue that contains a compound--melatonin--that has a potent skin lightening effect in certain amphibians and reptiles, via its effect on dermal melanophores. The physiology of the mammalian pineal was completely unknown, and it was often considered to be a "rudimentary" tissue in this group of vertebrates. Research accomplished since that time has clearly established that the pineal hormone, melatonin, has an essential role in the photoperiodic mechanism of mammals. In this role, melatonin has potent effects on a wide variety of seasonal, photoperiod-influenced traits; these include reproduction, pelage, thermoregulatory behavior, body mass, and lipid content. There is also some evidence for a role of melatonin in the circadian system of mammals. Through much is known about the overt actions of melatonin, little is known about he specific sites of action of this hormone. Using high specific activity radiolabeled melatonin, several sites of uptake have recently been identified, but there is no firm evidence for the specific role of any of these sites with respect to the actions of melatonin. This proposal is a plan to investigate the action of melatonin at three of these uptake sties--the suprachiasmatic nuclei, the nucleus reuniens, and the paraventricular nuclei of the thalamus, and also in the anterior hypothalamus, and the mediobasal hypothalamus, which have been suggested as melatonin target sites. Melatonin will be administered locally at each of these CNS nuclei via microdialysis, and photoperiodic and circadian responses will be monitored. Siberian hamsters will be used in these studies. A good deal of useful background information exists for this species, and my laboratory is experienced in studying photoperiodic and circadian mechanisms in Siberian hamsters.